Macromolecules 2002, 35, 1591-1598
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Synthesis and Polymerization of Methyl 3-Methylcyclobutene-1-carboxylate Tatsuki Kitayama,*,† Takehiro Kawauchi,† Nori Ueda,† Carina S. Kniep,‡ Won Suk Shin,‡ Anne B. Padias,‡ and H. K. Hall, Jr.*,‡ Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan; and Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Received April 20, 2001
ABSTRACT: A new cyclobutene monomer, methyl 3-methylcyclobutene-1-carboxylate (MMCB), has been synthesized and polymerized by radical and anionic initiators. The synthesis started from a [2 + 2] cycloaddition of N-(1-propenyl)piperidine to methyl acrylate, followed by methylation and treatment with base to yield the monomer. Free radical polymerization of MMCB led to low yields of low molecular weight polymers, probably due to chain transfer at the allylic hydrogen. However, p-methoxystyrene, styrene and methyl methacrylate gave high molecular weight copolymers with MMCB in high yields. Anionic homopolymerization with tert-butyllithium (t-BuLi), t-BuLi/bis(2,6-di-tert-butylphenoxy)ethylaluminum, lithium bis(trimethylsilyl)amide, and potassium bis(trimethylsilyl)amide in toluene at 0 and -78 °C proceeded smoothly and gave polymers in high yields. NMR and IR analyses of the polymers suggested that the polymerization proceeds via addition mechanism without ring-opening. Thermal properties of the polymers are also described briefly. In contrast to MMCB, methyl 3,3-dimethylcyclobutene-1carboxylate MDCB did not polymerize due to excessive steric hindrance.
Introduction
Scheme 1
Polymers containing 1,2- and 1,3-linked cyclobutane rings carrying a functional group are of interest for their good physical properties.1-4 They can be considered for use in specialty high-tech applications. Wider use of such polymers has been limited because of difficulty in obtaining sufficient supplies of monomers by easy syntheses. The main problem is the formation of a suitably functionalized cyclobutane. Among the common rings, the cyclobutane ring is the most difficult to form. However, Brannock et al.5 reported very promising results from the [2 + 2]-cycloaddition reactions of enamines with electrophilic olefins. Cycloaddition of enamines possessing no β-hydrogen, such as N,Ndimethylisobutenylamine, to acrylonitrile, methyl acrylate, and diethyl fumarate led to an extensive series of functional gem-dimethylcyclobutanes. They were also able to obtain a second series of monomethyl cyclobutane derivatives from cycloaddition reactions with enamines that did possess a β-hydrogen, such as N,Ndimethylpropenylamine. Particularly interesting to us was the reported conversion of these cycloadducts to cyclobutenes by quaternization with methyl iodide, followed by treatment with base. Examples (Scheme 1) are methyl 3-methylcyclobutene-1-carboxylate (MMCB), methyl 3,3-dimethylcyclobutene-1-carboxylate (MDBC), dimethyl 4,4-dimethyl-1-cyclobutene-2,3-dicarboxylate, 3,3-dimethylcyclobutene-1-carbonitrile, and 3,3-dimethylcyclobutene-1-carboxylic acid. We have been unable to find any reports of polymerization studies of these cyclobutenes. Among the potential cyclobutene monomers, we selected two monomers for further study, MMCB, with only one β-methyl group, and methyl 3,3-dimethyl-1-cyclobutenecarboxylate (MDCB). Steric repulsions between monomer and grow† ‡
Osaka University. University of Arizona.
ing polymer chain might be substantial. The monomer carrying a single methyl group could react with growing radical or anion on the face away from the methyl substituent, but steric hindrance in MDCB might be too much. Accordingly, we set about establishing practical syntheses of these monomers within the framework of the reported synthesis5 and studied their polymerizations using both radical and anionic initiation. Experimental Section Measurements. NMR spectra were recorded on a Varian Gemini 200 or a Varian Unity Inova 500 spectrometer. The IR spectra were obtained on a Nicolet Impact 410 infrared spectrometer or a JASCO Herschel series FT/IR-410 spectrometer. Mass spectrometric low-resolution data (EI) were recorded on a HP 5988A GC/MS system. All high-resolution mass spectra including fast atom bombardment (FAB) with highand low-resolution were taken on a JEOL HX 110A sector instrument. The elemental analyses were performed by Desert Analytics, Tucson, AZ. Molecular weight and molecular weight distribution were determined by size exclusion chromatography (SEC) using a JASCO 880-PU chromatograph equipped with Shodex SEC columns [KF-806L (30 cm × 0.8 cm) × 2] using chloroform as an eluent. The SEC chromatograms were calibrated against standard poly(methyl methacrylate) samples. Thermal analyses of the polymers were made on a SEIKO DSC 6200 and a SEIKO TG/DTA 6200. Materials. Methyl acrylate were purified prior to use by filtering over a column with basic aluminum oxide. MMCB, styrene, p-methoxystyrene, isobutyl vinyl ether, methyl acrylate, n-butyl acrylate, and methyl methacrylate were purified
10.1021/ma010683i CCC: $22.00 © 2002 American Chemical Society Published on Web 01/23/2002
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prior to polymerization by distillation over calcium dihydride under reduced pressure and in a nitrogen or argon atmosphere. Toluene was purified in the usual manner, mixed with a small amount of butyllithium, and distilled under high vacuum. 2,6Di-tert-butylphenol, obtained commercially (Tokyo Kasei Kogyo Co., Ltd.), was fractionally distilled, and used as a heptane solution. tert-Butyllithium (t-BuLi) in pentane (Aldrich Co., Ltd.) was used as a heptane solution. The concentration was determined by titration with butan-2-ol using o-phenanthroline as an indicator.6 tert-Butylmagnesium bromide (t-BuMgBr) was prepared from magnesium turnings and tert-butyl bromide in diethyl ether.7 The concentration of the t-BuMg group was determined by hydrolysis with excess 0.1 M HCl and back-titration with 0.1 M NaOH using phenolphthalein as an indicator. Lithium bis(trimethylsilyl)amide [LiN(SiMe3)2] as a hexanes solution and potassium bis(trimethylsilyl)amide [KN(SiMe3)2] as a toluene solution were purchased from Aldrich Co., Ltd. and used as received. LiN(SiMe3)2 concentration was determined by titration with butan-2-ol and that of KN(SiMe3)2 by acid-base titration. Bis(2,6-di-tert-butylphenoxy)ethylaluminum [EtAl(ODBP)2] was prepared from 2,6-ditert-butylphenol and triethylaluminum in heptane at 0 °C, according to the literature,8 recrystallized from heptane, and used as a toluene solution. N-(1-Propenyl)piperidine (1). The enamine was prepared by a condensation method described in the literature using anhydrous potassium carbonate.9 The primary amination product (aminal) synthesized from 1 molar equiv of propionaldehyde (70 g, 1.20 mol) and 2 molar equiv of piperidine (204 g, 2.40 mol), was thermally decomposed by distillation. Very slow fractionation through a 12 cm Vigreux column under an argon atmosphere in vacuo provided 74% (111 g, 0.89 mol) of 1 as a colorless oil (bp 99-102 °C, 75 mmHg).9,10 1H NMR (CDCl3): δ 5.86-5.79 (m, 3J ) 13.8 Hz, 4J ) 1.4 Hz, 1 H, CH(NR2)), 4.38 (dq, 3J ) 6.4, 13.8 Hz, 1 H, CH(CH3)), 2.75-2.70 (m, 4 H, R-ring-CH2), 1.62 (dd, 3J ) 6.4 Hz, 4J ) 1.4 Hz, 3 H, CH3), 1.65-1.41 (m, 5 H, β,γ-ring-CH2). 13C NMR: δ 141.6 (CH(NR2)), 94.5 (CH(CH3)), 50.2 (R-ring-CH2), 25.7 (β-ringCH2), 24.7 (γ-ring-CH2), 15.8 (CH3). IR (neat): 3052, 3033 (d CH2), 1694, 1660 (CdC) cm-1. MS (EI): m/z (rel intensity) 125 (M+, 42), 124 (M+ - H, 33), 110 (M+ - CH3, 100), 96 (18), 82 (14), 68 (25), 55 (10), 41 (21). HRMS (EI): calcd for C8H15N (M+), 125.1204; found, 125.1202. 1-Methoxycarbonyl-3-methyl-2-(N-piperidinyl)cyclobutane (2). Similar to a procedure described in the literature,5 enamine 1 (63 g, 0.50 mol) and methyl acrylate (43 g, 0.50 mol) in the presence of a trace of bis(3-tert-butyl-4-hydroxy5-methylphenyl) sulfide as an inhibitor were combined under an argon atmosphere and stirred at ambient temperature. After 3 days, conversion of the starting materials to cyclobutane 2 was quantitative. Distillation through a 12 cm Vigreux column under reduced pressure (bp 92 °C, 1.0 mmHg) gave 88% (93 g, 0.44 mol) of cyclobutane 2 as a mixture of at least 2 isomers (lit.:3 67%). 1H NMR (CDCl3): δ 3.69, 3.67 (2 s, 3 H, OCH3), 2.88-2.48 (m, 2 H, CH(NR2), CH(CO2CH3)), 2.41-2.08 (m, 6 H, CH2, R-ring-CH2), 1.64-1.30 (m, 7 H, CH(CH3), β,γ-ring-CH2), 1.13, 1.05 (2 d, 3J ) 6.2 Hz, 3 H, CH3). 13 C NMR (CDCl3): main isomer δ 174.8 (CO2CH3), 70.8 (CH(NR2)), 51.2 (CO2CH3), 50.7 (R-ring-CH2), 39.8 (CH(CO2CH3), 31.7 (CH(CH3)), 26.2 (CH2), 25.2, 24.0 (β,γ-ring-CH2), 20.7 (CH3). IR (neat): 1732 (CdO) cm-1. MS (FAB) m/z 212 (MH+), 126 (MH+(enamine)), 85. HRMS (FAB): calcd for C12H22NO2 (MH+): 212.1651; Found: 212.1650. Anal. Calcd. for C12H21NO2: C, 68.21; H, 10.00; N, 6.63. Found: C, 68.00; H, 10.15; N, 6.68. N-(1-Methoxycarbonyl-3-methyl-2-cyclobutyl)-Nmethylpiperidinium iodide (3). The quaternization of cyclobutane 2 (70 g, 0.33 mol) with methyl iodide (57 g, 0.40 mol) was carried out according to a method described in the literature.11 After 4 days at ambient temperature, methiodide 3 solidified and was isolated in 86% yield (100 g, 0.28 mol) by filtration. Recrystallization from ethanol/petroleum ether (4:1) gave 68% of 3 (79 g, 0.22 mol) as slightly yellow crystals (mp 108 °C). 1H NMR [dimethyl sulfoxide (DMSO)-d6]: δ 4.124.03 (m, 1 H, CH(N+(CH3)R2)), 3.66 (s, 3 H, OCH3), 3.62-3.55
Macromolecules, Vol. 35, No. 5, 2002 (m, 1 H, CH(CO2CH3)), 3.37-3.14 (m, 4 H, R-ring-CH2), 3.04 (s, 3 H, N+(CH3)R2), 2.95-2.65 (m, 1 H, CH(CH3)), 2.45-2.23 (m, 2 H, CH2), 1.93-1.35 (br m, 6 H, β,γ-ring-CH2), 1.16 (d, 3J ) 6.2 Hz, 3 H, CH(CH3)). 13C NMR (DMSO-d6): δ 171.9 (CO2CH3), 72.9 (CH(N+(CH3)R2)), 58.3, 58.1 (R-ring-CH2), 52.0 (OCH3), 43.4 (CH(CO2CH3)), 35.6 (N+(CH3)R2), 27.7 (CH(CH3)), 25.0 (CH2), 20.2, 18.9 (β,γ-ring-CH2), 19.7 (CH(CH3)). IR (KBr) 1737 (CdO) cm-1. MS (FAB): m/z 226 (MH+(cation)), 185, 93, 75, 57. HRMS (FAB): calcd for C13H25NO2 (MH+(cation)), 226.1807; found, 226.1809. Anal. Calcd for C13H24NO2I: C, 44.21; H, 6.85; N, 3.97. Found: C, 44.09; H, 6.90; N, 3.90. Note: Usually, crude cyclobutane 2 was used for the preparation of methiodide 3 without purification. Methyl 3-Methylcyclobutene-1-carboxylate (MMCB). Under an argon atmosphere, a 60% dispersion of sodium hydride in mineral oil (11 g, 0.27 mol) was washed twice with low boiling petroleum ether (25 mL each) and suspended in dry tetrahydrofuran (THF) (350 mL). Methiodide 3 (79 g, 0.22 mol) was added through a powder funnel in one portion. The reaction starts immediately as witnessed by evolution of hydrogen gas and a moderate temperature increase. After the mixture was stirred at room temperature for 1.5 h, the yellow color had faded and no more gas evolved. The reaction mixture was centrifuged, the solution decanted, and the residues were washed three times with dry THF, each time followed by centrifuging and decanting. Subsequently, a trace of bis(3-tertbutyl-4-hydroxy-5-methylphenyl) sulfide (inhibitor) and some dry ice were added to the combined THF solutions, which were then poured onto brine containing a small amount of ice. After separation of the aqueous layer, the organic one was washed with brine, whereas piperidine was removed by extraction with 6 M HCl. After this was washed with water, the colorless organic layer was dried over MgSO4, concentrated, and fractionated over a 6 cm Vigreux column under reduced pressure to give 71% (20 g, 0.16 mol) of cyclobutene MMCB as a colorless liquid (bp 70-71 °C, 15 mmHg). If inhibited with 3-tert-butyl-4-hydroxy-5-methylphenyl sulfide, MMCB can be stored for several months at -50 °C without decomposition. 1H NMR (CDCl ): δ 6.81 (s, 1 H, dCH), 3.73 (s, 3 H, OCH ), 3 3 2.92-2.75 (m, 2 H, CH(CH3), CHH), 2.28-2.15 (m, 1 H, CHH), 1.19 (d, 3J ) 7.1 Hz, 3 H, CH3). 13C NMR (CDCl3): δ 163.1 (CO2CH3), 151.5 (dCH), 136.5 (dC(CO2CH3)), 51.2 (OCH3), 36.5 (CH2), 34.9 (CH(CH3)), 18.2 (CH3). IR (neat): 3057 (Cd C), 1726 (CdO), 1611, 1601 (CdC) cm-1. MS (EI): m/z (rel intensity) 126 (M+, 34), 111 (36), 95 (24), 67 (100). HRMS (EI): calcd for C7H10O2, 126.0681; found, 126.0681. Methyl 2-(dimethylamino)-3,3-dimethylcyclobutanecarboxylate.5 A mixture of methyl acrylate (156 g, 1.82 mol), N,N-dimethylisobutylamine12 (60 g, 0.61 mol), and acetonitrile (250 mL) was refluxed overnight. Solvent and unreacted starting materials were removed by rotary evaporation. Vacuum distillation gave 61 g (yield 54%) of 2-(dimethylamino)-3,3dimethylcyclobutanecarboxylate, bp 60-65 oC at 2 mmHg. 1H NMR(CDCl3): δ 3.60 (s, 3H), 2.74 (q, 1H, J ) ∼9 Hz), 2.43 (d, 1H, J ) 8.7 Hz), 2.02 (s, 6H), 1.69 (dd, 1H, J ) 9.6. 10.5), 1.55 (dd, 1H, J ) 9.6, 10.5), 1.07 (s, 3H), 1.05 (s, 3H) ppm. 13C NMR(CDCl3): δ 157.2, 72.3, 51.6, 43.1, 39.3, 35.4, 33.4, 29.8, 21.7 ppm. N-(1-Methoxycarbonyl-3,3-dimethyl-2-cyclobutyl)N,N,N-trimethylammonium Iodide. Methyl iodide (63 g, 0.44 mol) was added to a solution of methyl 2-(dimethylamino)3,3-dimethylcyclobutanecarboxylate (61 g, 0.33 mol in 300 mL of diethyl ether), and the mixture was stirred for 6 days. Then the mixture was filtered and dried in a vacuum to give 89 g (83%) of N-(1-methoxycarbonyl-3,3-dimethyl-2-cyclobutyl)N,N,N-trimethylammonium iodide. 1H NMR (DMSO-d6): δ 4.00 (d, 1H, J ) 10.4 Hz), 3.76 (q, 1H, J ) 10 Hz), 3.64 (s, 3H), 3.09 (s, 9H), 1.91 (t, 1H, 10.2 Hz), 1.50 (t, 1H, J ) 9.9 Hz), 1.36 (s, 3H), 1.24 (s, 3H) ppm. 13C NMR (DMSO-d6): δ 171.9, 74.0, 52.1, 51.7, 40.3, 36.2, 32.8, 29.4, 22.5 ppm. Methyl 3,3-Dimethylcyclobutene-1-carboxylate (MDCB). Under an argon atmosphere, a 60% dispersion of sodium hydride in mineral oil (12 g, 0.30 mol) was washed twice with pentane (30 mL each) and suspended in dry THF (250 mL). N-(1-Methoxycarbonyl-3,3-dimethyl-2-cyclobutyl)-
Macromolecules, Vol. 35, No. 5, 2002 N,N,N-trimethylammonium iodide (65.4 g, 0.20 mol) was added through a powder funnel in one portion. After the mixture was stirred overnight at room temperature, anhydrous ether (200 mL) was added. Then the reaction mixture was centrifuged and the solution decanted, and the residues were washed three times with anhydrous ether, each time followed by centrifugation and decantation. Subsequently, a pinch of bis(3-tert-butyl-4-hydroxy-5-methylphenyl) sulfide (inhibitor) and some dry ice were added to the combined organic solutions, which were poured onto a mixture of ice and brine. After separation of the aqueous layer, the organic layer was washed with brine, 5 M HCl and distilled water. The organic layer was dried over MgSO4, concentrated, and vacuum distilled to give 18 g of cyclobutene. MDCB contained about 10% of a methanol adduct, methyl 3,3-dimethyl-2-methoxycyclobutane1-carboxylate, which was confirmed by GC-MS, but was used as such for the polymerization studies. 1H NMR(CDCl3): δ 6.78 (s, 1H), 3.67 (s, 3H), 2.38 (s, 2H), 1.17 (s, 6H) ppm. 13C NMR(CDCl3): δ 163.5, 155.4, 134.2, 51.1, 42.5, 40.9, 25.3 ppm. MS (m/z): 141 (100%, -CO2), 139, 125, 79, 53. Radical Homopolymerization in Bulk. Freshly distilled MMCB (0.005 mol), and AIBN (2 mol %) were degassed by freeze-thawing and kept at 80 °C for 20 h. The clear, colorless product was highly viscous at 80 °C and a glass at room temperature. The reaction product was dissolved in a small amount of dichloromethane and the polymer precipitated into cold diethyl ether. The colorless polymer was washed repeatedly with diethyl ether followed by drying over P2O5 at 80 °C in vacuo. The poly(MMCB) is soluble in dichloromethane, chloroform, acetonitrile and acetone. Radical Homopolymerization and Copolymerization in Solution. Solutions of freshly distilled MMCB (2.5 mmol) and AIBN (2 mol %) in N,N-dimethylformamide (DMF) or γ-butyrolactone (2.5 mL) were degassed by freeze-thawing. After 20 h at 80 °C, the reaction mixtures were poured into chilled diethyl ether. Isolation, purification, and drying of the polymers were carried out as described above. Polymerization in Aqueous Slurry. A mixture of freshly distilled MMCB (0.005 mol) in 3 mL of distilled water and aqueous solutions of K2S2O8 (0.1 M) and Na2S2O4 (0.05M) were degassed separately by freeze-thawing. Subsequently, K2S2O8(aq) (0.1 mL, 0.2 mol %) and Na2S2O4(aq) (0.1 mL, 0.1 mol %) were added to the stirred monomer slurry using a syringe. After 1.5 h, another 0.1 mL of each solution was added. Polymerization started after about 10-20 min, and the reaction mixture was stirred at ambient temperature for additional 20 h. The slurry was filtered and washed with water, and the colorless polymer was treated twice with water in a blender and one time in methanol, followed each time by filtering and washing. The polymer was dried over P2O5 at 80-100 °C in vacuo to give the product in 56% yield. Anionic Polymerization. All the anionic polymerizations were carried out in glass ampules, cooled at -78 °C and filled with dried nitrogen passed through molecular sieves 4A. The polymerization reaction was initiated by adding the monomer to the initiator solutions at the polymerization temperature and terminated by adding methanol containing a small amount of aqueous HCl.
Results and Discussion Monomer Synthesis. The route shown in Scheme 2 illustrates the successful synthesis of methyl 3-methylcyclobutene-1-carboxylate MMCB, which was also utilized for methyl 3,3-dimethylcyclobutene-1-carboxylate MDCB. N-(1-Propenyl)piperidine (1), from the reaction of piperidine with propionaldehyde as reported by Brannock et al.,5 was satisfactory as the enamine component. Cycloaddition of the enamine to methyl acrylate proceeded in high yield under moderate heating. Methyl iodide converted the cycloadducts to the quaternary ammonium salts. Precipitation of the salt from diethyl ether solution as it formed gave the pure salt as a mixture of isomers.
Methyl 3-Methylcyclobutene-1-carboxylate
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Scheme 2
Various methods to convert the salts to the cyclobutene monomers were examined. Brannock et al.5 used potassium hydroxide for this elimination, which resulted also in saponification of the ester to the carboxylic acid. In our hands, sodium methoxide gave the monomer containing a substantial amount of methyl 2-methoxycyclobutane-1-carboxylate, possibly formed by Michael addition of the methoxide to the formed monomer. Sodium hydride, a nonnucleophilic non-hydroxylic base, in THF was used at several temperatures. At room temperature (28 °C), the best results were obtained, i.e., 71% yield of MMCB, without hydrolyzing the ester functionality. It is interesting that the reaction of an insoluble base with an insoluble salt proceeds so readily. Radical Homopolymerization. The results of the free radical polymerization studies are summarized in Table 1. MMCB polymerized with AIBN initiation. However, neither the yields nor the molecular weights were high. The difficulty did not appear to be caused by impurities; rather we suspect that the tertiary allylic hydrogen acts as a chain transfer agent. In contrast to the homogeneous radical polymerizations, poly(MMCB) obtained with K2S2O8 and Na2S2O4 in a slurry polymerization in water, was not soluble in common organic solvents such as acetone, acetonitrile, chloroform, dichloromethane, DMF, DMSO, and THF. This is possibly due to cross-linking. To ascertain if spontaneous polymerization or isomerization of the monomer occurs under the polymerization conditions, MMCB was separately heated neat at 80 °C for 20 h. No polymerization occurred, and the yellow liquid obtained consisted mainly of the thermally stable monomer (>95%, 1H NMR, IR); i.e., no isomerization to 1,3-diene occurred. The dimethyl-substituted monomer MDCB did not polymerize under free radical conditions. In an attempt to overcome the expected low ceiling temperature, we tried the homopolymerization at -50 °C using redox conditions. Although these conditions smoothly polymerized methyl acrylate, no polymerization of MDBC was observed. The difference in reactivity between MMCB and MDCB is ascribed to the additional methyl group at the 3-position of MDCB. Whereas MMCB has an exposed face without steric hindrance, in MDCB both faces of the cyclobutene ring are sterically hindered by the two methyl substituents. Radical Copolymerization. Better results were obtained in copolymerization with electron-rich monomers. When p-methoxystyrene or styrene was copolymerized with MMCB, high molecular weight copolymers were obtained in high yields. Methyl methacrylate also copolymerized well with MMCB, but isobutyl vinyl ether gave only a low molecular weight product. The results
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Macromolecules, Vol. 35, No. 5, 2002 Table 1. Radical Polymerizations of MMCB with AIBN at 80 °C for 20 ha
run
solvent
initiator
1 2
yield/%
50
0 20
100 100
0 56
AIBN
3 4 a
monomer/initiator
DMF H2O
AIBN K2S2O8 + Na2S2O4
M h nb
M h w/M h nb
7900 58800 (shoulder)
1.34 2.31
Monomer 2.5 mmol; solvent 2.5 mL. b Determined by SEC in CHCl3, calibrated against PMMA standards.
Table 2. Radical Copolymerizations of MMCB and Several Comonomers with AIBN in Bulk at 80 °C for 20 ha isolated MMCB run comonomerb monomer/AIBN yield/% compn/%c 5 6 7 8 9 10 11 12
IBVE IBVE IBVE pMeOSt pMeOSt pMeOSt St MMA
0 67 56 0 67 56 47 48
25 50 25 50 100 50
Mwd
96 87
4900
